Prior art involving silicon resonant magnetometers suffer from poor sensitivity and moderate detection limits above 1 μT. In contrast, autonomous navigation systems that use position, navigation, and time units (PNTs) in conjunction with GPS applications need sensitive magnetometers wherein a minimum desired detection angle of 0.1° (in terms of resolving capability) translates to a minimum detectable magnetic field of under 50 nT.
Additionally, prior art silicon resonators are known to exhibit high temperature dependence, resulting in low stability of the measurement, whereas quartz resonators can be manufactured in various temperature-insensitive crystal cuts (AT cuts and SC cuts are preferred). Moreover, the high stability of co-fabricated MEMS quartz oscillators can be exploited to stabilize the measurement of the MEMS magnetometer. So a MEMS quartz oscillator may be advantageously disposed on the same substrate as the magnetometer disclosed herein.
The prior art includes: “Development of Miniature Magnetometers” Dennis K. Wickenden, Thomas J. Kistenmacher, Robert Osiander, Scott A. Ecelberger, R. Ben Givens, and John C. Murphy, Johns Hopkins APL Technical Digest, Vol. 18, Num. 2 (1997) 271.
This prior art achieves good detection sensitivity but utilizes a large (>4 mm) xylophone beam. High drive currents (>1A) prevent widespread use in portable and low power devices. The material of choice does not lend this prior art device to be easily integrated with electronics or other sensors. The present invention is based on a quartz MEMS fabrication process which is the subject of U.S. Pat. No. 7,830,074 identified above which has already demonstrated wafer-level integration with electronics.
The prior art also includes:
“A Resonant Micromachined Magnetic Field Sensor,” Behraad Bahreyni, and Cyrus Shafai, IEEE Sensors Journal, VOL. 7, NO. 9 Sep. 2007.
“A resonant magnetic field microsensor with high quality factor at atmospheric pressure,” A L Herrera-May, P J Garcia-Ramirez, L A Aguilera-Cortes, J Martinez-Castillo, A Sauceda-Carvajal, L Garcia-Gonzalez, and E Figueras-Costa, J. Micromech. Microeng. 19 (2009) 015016 (11 pp).
“Low Power 3-axis Lorentz Force Navigation Magnetometer,” M.J. Thompson, M. Li, and D.A. Horsley, MEMS 2011, Cancun, Mexico, Jan. 23-27, 2011.
“Micromechanical magnetometer using an all-silicon nonlinear torsional resonator,” D. Antonio, M. I. Dolz, and H. Pastoriza, Applied Physics Letters 95, 133505 2009.
The prior art mentioned above uses silicon resonant structures whose frequency shifts in the presence of a changing magnetic field. While silicon devices can easily be integrated with on-chip electronics, they are prone to large frequency-temperature drifts. Prior art often requires external and often separate drive and detection schemes which increases the complexities and noise. We design the drive and sense mechanisms directly onto the sensing structure enabling a more compact sensor footprint and reduced parasitics. Additionally, directly coupling between the drive and sense mechanisms of our invention greatly improves the signal-to-noise ratio.
Autonomous navigation and attitude heading referencing requires very precise magnetometers with detection limits under 50 nT to achieve an angle resolution of less than 0.1°. SOA magnetometers are forced to compromise between small volume/low power and detection sensitivity. The quartz magnetometer is designed to be one of the first micro-scale magnetometers that will break the 50 nT barrier without sacrificing CSWAP requirements.
In U.S. Provisional Patent Application Serial No. 61/943,213 filed 21 Feb. 2014 and entitled “A Micro-Scale Piezoelectric Resonating Magnetometer” we described a quartz piezoelectric magnetometer based on a rectangular plate or active region wherein the current drive and sensing electrodes are placed. Key to increasing the detection sensitivity of Lorentz force-based magnetometers is to concentrate or maximize the bending stress/ generated by the Lorentz force and position the sensing electrodes at or near the areas of greatest strain. The electrodes detect the strain as a shift in the resonant frequency of the normal quartz thickness shear mode oscillation. One direct method is to increase the Lorentz force that is applied along the front edge of the quartz plate during operation. Ignoring the current propagation along the top and bottom edges of the plate, the Lorentz electromagnetic force equation
            F      →        B    =            i      →        ⁢                  ⁢          w      tip        ×          B      →      is proportional to the current amplitude i, the magnetic field to be measured B, and the length of the current line which is roughly the width of the plate at the free end wtip in the above figure. Simply widening the entire quartz plate to increase the Lorentz force at the tip while maintaining a rectangular shape is not effective as the bending strain is independent of the plate width (the Lorentz force and the plate stiffness increase in proportion with each other, offsetting the effect of increased wtip).
U.S. patent application Ser. No. 14/628,182 filed 20 Feb. 2015 claims the benefit of the aforementioned U.S. Provisional Patent Application and includes additional disclosure including a new trapezoidal plate design where the plate width gradually grows from the base to the free end of the plate increases the strength of the Lorentz force while simultaneously concentrating bending strain at the sensitive thickness shear electrodes near the fixed end of the plate, resulting in enhanced sensitivity of the magnetometer to external magnetic field. See Appendix A of U.S. patent application Ser. No. 14/628,182 filed 20 Feb. 2015. Additionally, this new design is capable of detecting AC magnetic fields in addition to DC magnetic fields, within a certain bandwidth of DC (which bandwidth is estimated to be in the range of 20 to 30 Hz).
The plate need not necessarily be trapezoidal. What is important is that (a) the cantilevered plate have a certain width at or near its free end and (b) that the width of the plate at or near where the plate is supported be less than said certain width. With a cantilever design where one side is anchored and the other is free, it is better to maximize the force at the free end to obtain the greatest bending moment. The width is kept relatively narrow at the anchor side to maximize the bending stress.
An advantage of this design is that it is sensitive to multiple magnetic axes (i.e., a vector magnetometer). For example, FIG. 4 in the detailed description shows that this design can be used to detect both the Z axis magnetic field component (directed along the axis of the plate length) and X axis magnetic field component (directed transversely in-plane to the axis of the plate length). These two magnetic field components can be discriminated because of the frequency separation between the particular plate mode which captures the magnetic field component and their resulting modulation of the quartz thickness shear oscillation. That is, Z axis sensitivity is achieved because the Lorentz force along the plate width at the free end thereof induces the first flexural (diving board) mode of the quartz plate while the X axis sensitivity is achieved by the Lorentz force from an X directed magnetic field component excites the first torsional mode of the quartz plate. Because these modes are well-separated in frequency, it is possible to distinguish between each component of the magnetic field. By exploiting this principle further and optimizing the design of the magnetometer plate and current loop, it is conceivable that a full 3-axis magnetometer can be constructed using a single quartz MEMS device.